This invention relates to the field of optical to electrical signal transducer devices and their fabrication in the form of a field effect transistor (FET).
In providing semiconductor devices for transducing optical energy signals into electrical energy signals, it has become generally accepted that the field effect transistor provides something less than an optimum arrangement starting point for achieving the desired optical-to-electrical signal conversion or signal transducing. Although a number of effects contribute to this less than optimum starting point status, one especially significant consideration relates to the presence of optical energy-obscuring structure, including source, drain and gate-related metallic conductor members, dispersed over the same transistor surface used to receive an optical energy signal in the typical FET arrangement. In many FET arrangements these conductors, which communicate with the source drain and gate elements of the transistor, occupy such a major portion of the transistor's surface area as to relegate the remaining area usable for optical energy reception to less than 50% of the overall surface of the transistor.
In addition to this obscured surface area difficulty, when a GaAs FET structure is epitaxially grown onto a single crystal GaAs host substrate an interface is formed which creates a partial depletion of carriers at the interface. When the FET is fabricated this `built-in` depletion layer may act in opposition to the desired depletion region created by the Shottky barrier gate. This is not a severe problem for most typical applications of FETs, where the FETs are used for electrical amplification and modulation. If the FET is to be used as a photodetector, however, the "built-in" depletion region will undesirably limit the gain and modulation characteristics of the device. In addition, if the FET is inverted and the GaAs substrate removed, an air to active layer interface is created which also has a `built-in` depletion layer. This layer may actually enhance the achieved photoconductance since photons can be injected into the active layer.
Notwithstanding these phenomenon, there are a number of advantages in achieving an efficient photo transducing field effect transistor (i.e., a photo FET or OPFET). The availability of electrical signal amplification within the transducer device itself is, of course, one of the inherent advantages of a practical photo FET. The overall simplicity of a photo FET device and the possible fabrication material compatibility with integrated circuit devices are additional advantages to be achieved from an acceptable photo FET arrangement. In addition, the ability to perform processing on both sides of a photo FET the device provides an opportunity to create structures which have heretofore been impossible, for example, opposed-gate FETs and counter-electrode configurations, to tailor depletion regions and create ballistic (super fast) devices. Anti-reflection coatings, such as, silicon nitride, Si.sub.3 N.sub.4 ; silicon oxides SiO, SiO.sub.2 ; and wavelength filtering dielectrics can easily be applied to the exposed active layer using conventional evaporation or sputtering equipment. Moreover, passive and/or active devices, e.g., vertical cavity lasers (VCLs), can be bonded to the backside of such an OPFET to create an optical transceiver assembly.
The U.S. patent art indicates the presence of considerable inventive activity, especially with respect to the MESFET (metal semiconductor field effect transistor) device and its use as an electrical circuit element. Prior patent activity however appears limited in the area of photo optical transducing MESFET devices as is disclosed herein.
Previous MESFET related patents include the patent of U. K. Mishra et al., U.S. Pat. No. 5,180,681, which is concerned with the fabrication of a high current high voltage breakdown field effect transistor of the MESFET type, including the fabrication of such devices with gallium arsenide material. In addition the invention of S. Inmamura et al., in U.S. Pat. No. 5,204,278, discloses the use of Group III-V periodic table semiconductor compounds such as gallium arsenide and the configuration of a MESFET device to include a Shottky gate electrode arrangement. In addition, the invention of E. Kolesar Jr. in U.S. Pat. No. 4,989,063 is of general background interest with respect to the present invention in the sense that it discloses use of an epoxy adhesive during a semiconductor device fabrication sequence.
Neither the application of the MESFET configuration to optical transducing service nor the improved configuration of a MESFET device for this usage is known to have been accomplished however.